Hiroki Sekiya

Effect of epoxy cracking resistance on the stability of impregnated superconducting solenoids

The stability and quench positions of superconducting test solenoids impregnated with three types of epoxy resin, with different crack resistances, have been studied. Voltage signals and acoustic emission (AE) were monitored during training tests. Premature quenches in the solenoids almost occurred at the innermost layer of windings; voltage spikes and AE appeared just prior to the premature quench initiation. This suggested that premature quenches were due to friction or debonding between ground insulation and winding. The solenoid with a release film between ground insulation and winding showed high stability and reached critical current at the second charging. From the results, the effects of epoxy cracking resistance on the stability of impregnated superconducting test solenoids are discussed.

Effect of ground insulation contact on stability of epoxy-impregnated superconducting solenoids

To clarify the effect of ground insulation contact on the stability of epoxy-impregnated superconducting solenoids, test coils with ground insulation between a winding and a metal former were fabricated. Aluminium and stainless steel were used as the former materials. The training effects of coils and the circumferential strains of former inner surfaces were measured. The training effects of aluminium former coils were larger than those of the stainless steel former coils. The strains of stainless steel formers became greater as the exciting current increased, but the strains of the aluminium formers did not change. These results show that the insulation of aluminium former coils became a floating state, the insulation of stainless steel former coils maintained a pressed state, and the floating state caused the large training effects.

Preparation and characteristic evaluation of hydrophobic epoxy-based nanocomposites

Nanocomposite techniques are used to improve the electrical properties of epoxy materials, for example, insulation breakdown strength and partial discharge resistance. In the first step, we prepared epoxy-based nanocomposites by using hydrophobic epoxy as the base resin and 10wt% of titanium dioxide or 5wt% of layered silicate as nano fillers, and measured the partial discharge resistance and insulation breakdown strength. It was confirmed that the depth of the hole caused by partial discharge exposure was decreased to half and one-fourth and the insulation breakdown strength was increased by about 0.8% and 4% with the addition of titanium dioxide or layered silicate, respectively, as compared to the base resin. In the second step, we fabricated impregnated mica tapes, which have been widely used in high-voltage equipment, using nanocomposite epoxy as binder resins. We can see that, although the erosion depth was not changed significantly, the insulation breakdown strength was increased by about 35% and 8.7% in mica tapes containing titanium and layered silicate, respectively, compared with mica tape without nano-fillers.

The module type power device united with a heat sink

The size of inverter equipment using conventional module type power devices becomes large in the consequence of large steady-state and transient thermal resistance by the following two reasons. (1) Because of the curvature of the base plate, contact thermal resistance of the conventional module type power device is relatively large, and steady-state thermal resistance also becomes large. (2) Since module type power devices are manufactured aiming at low cost and low steady-state thermal resistance, generally their heat capacity becomes small and their transient thermal resistance becomes large. We propose a new structure of a module type power device. The structure is that a semiconductor chip soldered on an electrode is united with a heat sink by using an epoxy resin. It is confirmed that the steady-state and the transient thermal resistance of the proposed module type power device is decreased to about 50% compared to conventional type.

Application-oriented researches on nanocomposite insulating materials

In the past decade, the nanocomposite technology has made much progress rapidly. We also developed the epoxy-based nanocomposites which have the enhanced insulation properties. The advantages of the nanocomposite materials stimulate the efforts for practical use of them. Therefore, the applications of the nanocomposite materials are primary interests in current researches. In this paper, we briefly introduce the four kinds of the application-oriented researches on the nanocomposite insulating materials.